The present invention relates to an exhaust emission control apparatus arranged to electrically heat a catalyst and to supply secondary air to the catalyst in order to rapidly warm up it to a minimum effective operating temperature.
Japanese Patent Provisional Publication No. 7-11945 discloses an exhaust emission control apparatus including an electrically-heated catalytic converter and a secondary air supply device. The electrically-heated catalytic converter functions to rapidly warm up the catalyst by receiving electric power. The secondary air supply device supplies fresh air to exhaust gases so as to raise the temperature of the exhaust gases by exhaust thermal reaction and to improve the oxidation reaction power of the exhaust gases at the catalyst. This conventional exhaust emission control apparatus simultaneously executes these two-way treatments of exhaust gases.
However, such simultaneous execution causes several problems. For example, since the electrically-heated catalytic converter consumes a large amount of electric power such as 2 Kw to 3 Kw during the electric heating, it is necessary to increase the generating voltage of an alternator for supplying electric power to the converter during the electric heating. Such load increase of the alternator increases an engine load and therefore the intake air flow is also increased. This increase of the intake air flow increases the passing speed of the exhaust gases through an exhaust gas passage, and therefore the thermal reactor effect to be ensured by the secondary air supply is decreased as shown in FIG. 13. Accordingly, the performance of the catalyst is lowered such that the discharged amount of the un-burnt HC and CO increases. Further, the increase of the flow amount of the exhaust gases due to the increase of the intake air amount increases the discharged amount of the un-burnt HC and CO.